Part of a major road has collapsed in Adelaide because of a burst water main, leading to peak-hour traffic chaos.

With parts of the city already affected by road closures due to the V8 Supercars event in the east park lands, two lanes on Greenhill Road near the south park lands were closed off because part of the roadway caved in.

For much of the day just one eastbound lane was open on Greenhill Road, near the corner of Unley Road.

Water gushed across the road and a footpath into the park lands for some time before emergency crews managed to switch off the main.

Steve Rose from SA Water said repair work could take much of the day.

"Even once the main itself is repaired and water is restored, it will be quite some time before the road is actually fully restored to traffic," he said.

So much water flowed from the breach in the pipe that the south park lands had what appeared to be an instant billabong.

You have no doubt been hearing a lot about the Paris Agreement and know that it pertains to climate change, but are too embarrassed at this stage to ask for an overall explanation of what it's all about.